This invention relates to cartons, and especially to cartons for use in the storage of stacks of paper in sheet form for use in xerographic, magnetographic, ion deposition, and especially laser xerographic imaging processes.
The conventional xerographic process for producing copies from a master on plain paper operates by forming on a suitable receptor surface an optically generated electrostatic negative image of the document to be copied, dusting the image with a fusible coloured toner, transferring the toner to form a positive image on a plain paper sheet and fusing it to the sheet by the application of heat.
The xerographic process will not operate satisfactorily with paper having too high a moisture content, and the moisture level of the paper supplied for use in the process must therefore be subject to careful control. Similar moisture constraints apply to paper for use in ion deposition or magnetographic printing processes. Typically a ream of such paper would be packed in a paper wrapping having a moisture vapour transmission rate (MVTR) of not more than 10 grammes per square metre in 24 hours to relative humidity of 90% and a temperature of 32.degree. C.
Since such papers are used for copying at a substantial rate, and a ream of such paper, once opened, will not generally remain unused for a long period, the wrapping can generally be discarded within a short time after the pack has been opened. The resulting relatively short term exposure to ambient conditions will not generally cause excessive moisture absorption before the paper is used.
In a recent modification of the xerographic process however, the electrostatic image is not formed optically from a master, but by means of a laser driven from the memory of a computer or word processor. The laser thus reproduces as the electrostatic image the typed material contained in the memory of the machine. Laser xerographic printing can thus be substituted for impact or ink jet printing as currently used for producing hard copy derived from computers or word processors. As a result, laser xerography can be used in substitution for a conventional typewriter for printing an individual letter on preprinted letterhead paper. Unlike conventional xerography therefore, the sheets from a pack of paper will usually be fed into a laser xerographic printer intermittently and at a much lower frequency than in conventional xerography - being used by secretarial staff at substantially the same rate as in producing individual items of conventional typed correspondence.
A pack of paper of, say 500 sheets, for use in laser xerographic printing, may therefore be of much higher quality than that generally used in conventional xerographic copying, but may be used at a much slower rate, with a few sheets at a time being removed intermittently from the pack for use. Paper suitable for such use is described in copending U.S. Patent Application No. 07/295,375 (corresponding to United Kingdom Patent Applications Nos. 88 01044 and 88 17113) which require inter alia that the paper be maintained to the point of use at a moisture content below 6%.
The paper must therefore be manufactured at a moisture content in contemplation of the likely moisture take up during storage and intermediate processing, such as preprinting, and the packaging must be capable of maintaining the paper at or below the maximum permitted moisture content. At the same time, the packaging must be capable of relatively airtight reclosure after initial opening so as to minimise moisture ingress whilst in a closed condition during the possibly substantial period during which complete usage of the paper will occur. It is among the object of the present invention to provide a reclosable pack of paper intended for use in laser xerographic or similar reproduction, which affords adequate resistance to moisture ingress so as to maintain the paper in good condition in such reproduction, and which, optionally, can be repeatedly resealed.